The present invention generally relates to an electron beam apparatus which is demountable and a method for producing a high electron beam current which has a low energy spread at high brightness and a uniform intensity distribution.
Many different demountable electron beam apparatus have been developed in the prior art for the purpose of producing an electron beam having a low energy spread at high brightness by using a sharp tip of thermionic cathode such as lanthanum hexaboride (LaB.sub.6) or using a sharp tip of field emission cathode. These cathodes are primarily used for producing a low beam current (typically less than several nanoamperes). A field emission cathode has difficulties for producing a high beam current. A lanthanum hexaboride cathode with a sharp tip produces an electron beam with a non-uniform intensity distribution. In electron beam apparatus wherein a shaped beam is used in microfabrication of large scale integrated circuit patterns as described in U.S. Pat. No. 4,213,053, U.S. Pat. No. 4,243,866 and U.S. Pat. No. 4,163,155, a high beam current having a low energy spread in a focused shaped beam on a target is essential for fast writing time of the circuit patterns (typically more than several microamperes). Other desirable features for such a focussed shaped beam are a high current density, a uniform distribution of current density and a sharp beam edge. These features may be attained by illuminating a spot shaping aperture with an electron beam which has a low energy spread at high brightness and a uniform intensity distribution.
It can be clearly understood how the high brightness, the low energy spread and the uniform intensity distribution are related to the high current density, the uniform distribution of current density and the sharp beam edge of a focussed shaped beam with the following relation: EQU J.perspectiveto.J.sub.o (eV/.DELTA.E.sub.t) sin.sup.2.alpha.
This relation gives the current density J attainable in a focussed shaped beam. The saturation emission current density is J.sub.o, the beam voltage is V, the electronic charge is e, the absolute temperature is T, the half beam convergence angle is .alpha., and the transverse energy spread is .DELTA.E.sub.t. If .DELTA.E.sub.t is equal to kT (k is Boltzmann constant), the above relation becomes Langmuir's relation. The brightness (.beta.) of the beam is given by .beta.=J.sub.o /.pi.(eV/.DELTA.E.sub.t). The higher the saturation emission current density J.sub.o and the lower the transverse energy spread .DELTA.E.sub.t is, the higher the brightness (.beta.) becomes for a given value of beam voltage V. In turn, the higher brightness provides the higher current density J for a given value of beam convergence angle .alpha.. It is a well-established fact that, in a high current electron beam apparatus, the energy spread in the beam is substantially higher (as high as 10 eV) than the thermal energy spread (less than 0.5 eV). The increase in the energy spread is attributed to the electronelectron interaction. The increased energy spread causes not only the blurring of the beam edge in a focussed shaped beam but also the decrease in the attainable current density J. Furthermore, in an electron beam apparatus which uses a sharp cathode tip or tungsten hairpin cathode, the uniform current density is obtained by only accepting spatially and angularly central portion of the emitted electrons from the cathodes. Such a large unused emission current is very detrimental since the electrons in the unused emission current interact not only with themselves but also with those in the used emission current, thus broadening the energy spread in the used beam. The ratio of a beam current in a focussed spot to a total emission current, which is known as the current efficiency E.sub.c is typically less than 0.2% for 5% uniformity of the current density for an electron beam apparatus using a sharp cathode tip or tungsten hairpin cathode.
Therefore, an electron beam apparatus for producing an electron beam with a high current efficiency (E.sub.c), which has a low energy spread at high brightness and a uniform intensity distribution, is essential to attain a high throughput and high resolution in an electron beam lithography machine.